Personnel plutonium monitor

ABSTRACT

A system for monitoring a zone for the presence of a source of plutonium in which the updated background level of gamma rays is compared with the readings obtained when a possible carrier of a gamma ray source enters the zone. Counting is initiated, and an alarm is sounded when the counting after a predetermined period of time accumulates to a value in excess of the background total plus a significant statistical deviation as a factor of the background total.

[ 1 June 13, 1972 United States Patent Hardy et al.

References Cited PERSONNEL PLUTONIUM MONITOR [72] lnventors: Richard W.Hardy,

B. Knowlen, Goleta; Cecil W. Sandifer; William C. Flake, both of SantaBarbara, all of Calif.

Primary Examiner-James W. Lawrence United slat of America 95 AssistantExaminerDavis L. Willis represented by the United States Atomic R |a dA. Anderson Energy Commission Aug. 18, 1970 [21] Appl.No.: 64,713

The

[73] Assignee:

[57] ABSTRACT A system for monitoring a zone for the presence of asource of [22] Filed:

plutonium in which the updated background level of gamma rays iscompared with the readings obtained when a possible carrier of a gammaray source enters the zone. Counting is in- .250/83.3 R ....G01t 1/16.250/7l.5 R, 83.3 R, 83.6 R

[52] 'U.S.Cl.

itiated, and an alarm is sounded when the counting after a predeterminedperiod of time accumulates to a value in excess 511 Int.Cl............

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of the background total plus a significant statistical deviation as afactor of the background total.

5 Claims, 2 Drawing Figures PATENTEDJUH 13 m2 SHEEI 10F 2 INVENTORS.

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IMER 54v INVENTORS. 2 RICHARD w. HARDY ROBERT B. KNOWLEN CECIL W.SANDIFER WILLIAM C.PLAKE a, z 4, W

PERSONNEL PLUTONIUM MONITOR SOURCE OF THE INVENTION The inventiondescribed herein was made in the course of, or under a contract with theUnited States Atomic Energy Commission.

BACKGROUND OF THE INVENTION Based upon economic as well as strategicconsideran'ons, the control and security of nuclear materials isimportant to all parties responsible for handling and using them. One ofthe materials subject to this degree of concern is plutonium which,aside from its significance from the point of view of national security,is employed in appropriate form for the manufacture of fuels for certaintypes of nuclear reactors.

In facilities which handle or process this material the plutonium ispresent largely in the metallic, oxide, fluoride, or nitrate form with afew percent abundance of Pu in Pu The particular area of concern withrespect to these isotopes found to be the most difficult securityproblem is that of the unauthorized removal of plutonium in relativelysmall amounts, i.e.,down to 0.1 gram of Pu carried by plant personnel.The extent of the monitoring problem is indicated by the trafficencountered in one typical installation, that of as many as 700 personsa day with a peak rate of 60 persons per minute passing through standard3-foot-wide doors and available for observation for about 1 to 1.5seconds. Physical examination of all those persons on a regular basis istime consuming and inefiicient while the difficulty of monitoring byconventional radiation detection instruments is indicated by the factthat these materials produce passive radiation with the bulk of theenergy being alpha emissions which can be eflectively shielded orblocked by the thinnest and lightest of material. Gamma radiations whichare more penetrating, peak out at levels which are about percent of thealpha energies. Conventional gamma detection instruments aresuffieiently sensitive to sense small amounts of plutonium. However,conventional techniques are based on count rate measurements whichrequire either a decrease in effective sensitivity or an increase in thefalse alarm rate.

BRIEF SUMMARY OF THE INVENTION The present invention overcomes the majordeficiencies of presently available passive radiation detection systemsfor the monitoring of personnel to detect the presence of plutonium ingram quantities. It provides a unique arrangement capable of detectinggamma radiation at low energy levels and at counting rates heretoforeconsidered to be incapable of being attained while maintaining a lowfalse alarm rate.

In accordance with the principles of this invention, a preferredembodiment of the invention employs a small cluster of spaced gamma raydetectors, a summing circuit to combine the sensed information, andprovision to accumulate or integrate the information received during theperiod the subject is present in the zone of detection. During the timewhen no subject is present within this zone, the invention maintains anup-to-date reading of background gamma information for the same energylevels. When the subject enters the zone of detection, backgroundinformation is frozen and maintained until counting of the subject iscompleted. Background information is then compared with the informationtaken from the subject in accordance with a predetermined relationshipwhich can be altered somewhat by the operator to accommodate localconditions and instant requirements. Based upon this comparison thepresence of plutonium may be indicated and an alarm sounded if desired.

In addition to obtaining high plutonium sensitivity, a system embodyingthis invention has the additional advantages of simplicity of operationmaking it feasible for useby people of limited skills, minimuminterference with plant operation and personnel traffic, and low capitalinvestment. Good system reliability and the elimination of costlymaintenance normally associated with one-of-a kind components areobtained as a result of utilizing as the components of the systemreadily available commercial equipment.

It is thus a principal object of this invention to provide a personnelmonitoring system for plutonium sensitive to gram amounts of thematerial.

Other objects and advantages of this invention will hereinafter becomeevident from the following description of a preferred embodiment of thisinvention.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is an isometric view of adoorway arrangement for the detectors utilized in a preferred embodimentof this invention; and

FIG. 2 is a block diagram of a system embodying the princi ples of thisinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENT The plutonium to be monitored inaccordance with this invention appears as largely Pu with a few percentof Pu present in metallic, oxide, fluride or nitrate form. As shown inthe table, measurement of passive gamma radiation would be mosteffective at 375 and 414 KeV for Pu Experiments and studies have shownthat at these energies the gamma radiation will penetrate moderateshielding. By moderate shielding is meant shielding equivalent to leadthickness not in excess of 1-2 cm. It is assumed that for the detectionof plutonium carried in shielding of greater equivalent thickness normalvisual surveillance procedures provide adequate security.

1 MeV (complex). 2 Fission spectrum.

For a description of the overall system of a preferred embodimentutilized for monitoring a doorway reference is made to FIGS. 1 and 2.Through doorway opening 10 pass the personnel or subjects undergoingmonitoring. As will be described later, initiation of counting ofsubject is begun upon depression of floor mat switch 12 the length ofwhich would be such as to provide the normal time span pointed out belowfor a subject to cover in walking its entire length.

In the particular embodiment, four detectors 16a, 16b, 16c, and 16d aremounted flush in the side walls of opening 10 so that only the entry toeach detector is exposed. These detectors would be encased in shieldingentry to each detector as to have a 21rdirectional response as indicatedby broken lines 18a, 18b, 18c, and 18d efl'ectively enhancing thesource-tobackground ratio. The use of four detectors reduces averagedetector-source separation thereby increasing sensitivity of the system.Subject 22 is carrying a plutonium sample 24.

Detectors 16 a d are conventional gamma counters and may be NaIscintillation devices coupled to photomultipliers to view and respond tothe photon energy spectra from the P 23W240 As briefly pointed outabove, the system functions to count integrally and to compare with acontinuously updated background level. The comparison is automaticallymade at a preselected number of standard deviation intervals.Mathematically, the system solves the relation S,,-=source counts Bbackground counts B, stored background counts I B, standard deviation ofbackground counts N integral multipler, for selecting the number ofstandard deviation intervals considered to be significant.

In other words, the above mathematical statement becomes true andplutonium is presumed to be present on the subject when the cumulativecounts taken from the subject during the prescribed interval exceeds thesum of the stored background counts for a like interval and thepreselected number of standard deviations of the stored backgroundcounts.

Referring to FIG. 2 for the system for obtaining relation I) andcarrying out the principles of this invention, it will be seen that theoutputs of detectors 16a 16d are summed in amplifier 26 and amplified inamplifier 28, single channel analyzer (SCA) 32 selecting and passing onpulses representing the particular channel selected. Pulses fromanalyzer 32 are fed to a monostable multivibrator 34 whose amplitude isfixed and whose width is adjustable depending on the setting of rangeswitch 36. The output of multivibrator 34 is passed first through alow-pass filter 38 whose IO-second time constant is long compared to thesecond or so that the subject is in the counting area but short comparedto any changes in average background count rate. This produces an analogvoltage that is proportional to the average background rate. A digitalvoltmeter (DVM )42 reads this output voltage and displays the results.

When a subject enters the area or zone of counting, switch 12 is steppedon and actuated, causing a flip-flop 44 to place a hold on DVM 42 at itslast reading which is for the purpose of subsequent computation theaverage stored background B Digital-to-analog converter (D/A) 46continuously produces a negative analog voltage proportional to thestored voltage so that when switch 12 is depressed the negative voltagethen produced by D/A 46 is representative of the average backgroundgamma condition.

As the same time that switch 12 sets flip-flop 44, it also initiates8-second timer 48, reset timer 52, and -second timer 54, the purposes ofall of these to be explained below.

A data integrator 56, which is reset to zero by reset timer 52 when matswitch 12 is closed, receives positive pulses through resistor R frommultivibrator 34 representing, during the counting period just begun,background plus possible signal counts (S B,) from the subject. Thesepositive pulses cause integrator 56 to produce a stair-step count in thenegative direction (due to a polarity reversal). At the same time as thepreceding occurs, the stored background data from D/A 46 combined withthe output of a sigma circuit 57 to be described, in the form of asteady DC negative voltage by way of leads 58 (including balancerheostat 62) and 59, causes integrator voltage to rise as a linear rampin the position direction. Those two-inputs cause the output voltagefrom data integrator 56 to average zero. Since the background pulse rateis random the output voltage will look like noise with an average DCvalue of zero. This will cause red and green output lights 64 and 66,respectfully, to flash alternately.

It will be seen that stored background current is fed by lead 59 tointegrator 56 as already noted from sigma circuit 57 consisting of apassive diode squareroot circuit 68 and a two part attenuator circuit72. The latter is provided with a range switch S to set sigmaproportional to the range being used and a switch S to select the number(N) of sigmas to be added.

Date integrator 56 serves the dual purpose of accepting the differencebetween data pulses (S 8,) from multivibrator 34 and the stored andmodified background counts (B +N V B',) from D/A 46 and sigma circuit57, and integrating the results. The time of integration is notcritical, but longer times gives more accurate results.

The output of integrater 56 is fed to a comparator 74 which continuouslydetermines the polarity of the result. When the subject leaves thecounting area (releasing floor mat switch 12) the comparator output isstrobed. If the output is negative (as noted, there is a polarityreversal in integrator 56) when mat switch 12 is released the signal ispassed through and AND circuit 76 and alarm 78 is sounded.

Twenty-second timer 54 is designed to indicate sticking of mat switch12. If mat switch 12, upon being closed by a subject, does not reopenwithin 20 seconds, then alarm 78 is sounded indicating the possibilityof a failure, or the placement of a heavy object on the match switch 12.

The system just described operates in the following manner. When nosubject is within the monitoring area, DVM 42 continuously displays andupdates the average background count rate.

When a subject steps on mat switch 12, flip-flop 44 holds" the output ofDVM 42, and data integrator 56 is reset to zero. Output counts fromanalyzer 32, combined with the stored average background count fedthrough D/A 46, squared in circuit 68, multiplied by N in attenuatorcircuit 72 and added to the output of D/A 46 is fed into integrator 56,causing the latter to integrate the difference. Thus integrator 56subtracts the stored and modified background counts B N B, from the datapulses S, B and integrates the difference. Switch S, in attenuatorcircuit 72 may be used to alter the multiple N to obtain the degree ofselectivity desired.

Comparator 74 receives the output of integrator 56 and determinescontinuously the polarity of the result. When the subject leaves thecounting area as indicated in AND" gate from switch 12, the output ispassed to alarm 78 which is designed to go off when the output isnegative.

Background updating occurs at all times that there is no subject onfloor mat switch 12. This automatic feature is important for any suddenchanges in background due to movement of active materials in theneighborhood. The pulses are averaged by low-pass filter 38. Theaveraging period has no maximum value but would have a minimum time,such as 5 seconds. If mat switch 12 is open for less than this period,DVM 42 will not be strobed and the previous value will be used.

Rheostat 62 is used as a balance control which equalizes the electricalresponse of the low-pass filter 38, DVM 42, and D/A 46. Consider asteady train of pulses at the input multivibrator 34. Current from themultivibrator flows through 38, 42, 46, 62, 58 and then to theintegrator 56. Current also flows from the multivibrator 34 throughresistor R directly to the integrator 56. Balance rheostat 62 is used toinsure that these currents are equal in magnitude although opposite insign.

The output on connector 82 is used to monitor the short term behavior ofthe integrator. Such things as analytically monitoring the count rateduring a counting period or adjustment of rheostat 62 can beaccomplished.

It will be seen that the system as just described, which utilizescommercially available components, functions with little or no attentionas there are no critical adjustments to be made during its normaloperation. The system is readily adaptable to regular field use as itfits in with normal plant routines. The system will detect unshieldedplutonium samples in sizes from very large down to less than 1 gram. Itis unobtrusive and will provide a positive warning signal for use byplant personnel in controlling unauthorized plutonium removal.

In the event that an organized theft operation is suspected or to beprotected against, in which larger quantities of plutonium in well gammashielded containers would be carried out, a neutron monitoring systemcan be integrated with the described system. Detectors sensitive toneutron radiation would be added along with an electronic system similarto the gamma detection system described. In the neutron system theminimum source size detectable might be as low as 17 grams. Any neutronshield capable of effectively attenuating the spontaneous fission and(a,n) neutrons from the Pu sample will be large and easily detected byvisual inspection.

It is thus seen there has been provided a unique system for thepersonnel monitoring on a large scale of clandestine diversion of smallamounts of plutonium. The system as described which is sensitive to gramquantities or less of plutonium makes it unprofitable and risky foranyone to expend the labor and time required for the unauthorizedremoval of an appreciable amount of this material.

What is claimed is:

l. A gamma ray source detection system for use in a monitoring zonecomprising:

a. means for measuring and maintaining continuously the updatedbackground count rate of gamma rays in said monitoring zone;

b. means responsive to the movement of a possible source of gamma raysinto said monitoring zone for holding the then background count rate;

c. means for producing a first signal corresponding to the sum of theheld background count rate and a preselected factor of said heldbackground count rate;

(1. means for taking a second signal of polarity opposite to that ofsaid first signal from said measuring means corresponding to the countrate of gamma rays in said zone;

e. means also responsive to the aforesaid movement for cumulativelycounting the sum of said first and second signals; and

f. means responsive to the movement of said possible source of gammarays out of said monitoring zone for indicating that said possiblesource is an actual gamma source when said cumulatively counting meansproduces an output corresponding to the same polarity as said secondsignal.

2. The detection system of claim 1 in which said indicating meansdelivers an alarm signal warning of the presence of a gamma source whensaid source leaves said zone.

3. The detection system of claim 1 in which the preselected factor is amultiple of the square root of said held background count rate.

4. The detection system of claim 3 in which the means responsive to themovement of a possible source of gamma rays includes a switch initiatedby said movement, the release of said switch when said possible sourceleaves said zone causing termination of the aforesaid cumulativecounting and indicating of the presence of the gamma ray source.

5. The detection system of claim 4 having means to change said multiple.

2. The detection system of claim 1 in which said indicating meansdelivers an alarm signal warning of the presence of a gamma source whensaid source leaves said zone.
 3. The detection system of claim 1 inwhich the preselected factor is a multiple of the square root of saidheld background count rate.
 4. The detection system of claim 3 in whichthe means responsive to the movement of a possible source of gamma raysincludes a switch initiated by said movement, the release of said switchwhen said possible source leaves said zone causing termination of theaforesaid cumulative counting and indicating of the presence of thegamma ray source.
 5. The detection system of claim 4 having means tochange said multiple.